1. Field of the Invention
The invention relates to an operational amplifier circuit and, more particularly, to a complimentary metal oxide semiconductor (CMOS) operational amplifier output stage which can be connected to almost any type of input stage design.
2. Description of the Related Art
Bipolar devices for operational amplifier output stages do not yield good characteristics in sub-micron dimensions; the current gain is low resulting in an input impedance that is not high enough and the base current is significant, thus loading the output of the first stage of an operational amplifier. An operational amplifier output stage using purely CMOS devices which overcomes several problems is the better approach.
An example of the prior art is shown in FIG. 1 and is discussed in U.S. Pat. No. 5,442,320 (Kunst et al.) as FIG. 1 Prior Art. The patent discloses a Class AB output stage using bipolar transistors providing a large swing and rail-to-rail output drive. FIG. 1 of this patent is a prior art schematic circuit diagram of an operational amplifier output stage and illustrates a two current source design. Problems that this design has, and which become more severe as device dimensions shrink, is that the base current of transistor 122 loads the output of the first stage. In addition to this, the circuit is biased with two independent current sources 150 and 152. These two current sources have to match very well in order to have a low input offset voltage.
U.S. Pat. No. 5,293,136 (Ryat) discloses both bipolar and MOS embodiments of a 2-stage rail-to-rail operational amplifier that requires an RC network for both output transistors and where the output of the first stage is not directly coupled to an output transistor of the output stage.
U.S. Pat. No. 5,546,045 (Sauer) describes a rail-to-rail Class AB operational amplifier output stage that has only bipolar implementation, differs in circuit detail and requires frequency compensation.
U.S. Pat. No. 5,568,090 (Pierret) discloses an operational amplifier output stage which is oriented to increasing the output drive current by sensing that current and adjusting a bias current. Only a bipolar implementation is described.